Fish inhabit boreal lakes throughout the year and aquatic invertebrates living in the same lakes belong on the menu of several fish species. Ducks also utilize these same invertebrates for half the year. Even ducks that usually eat plants consume invertebrates; especially females preparing to lay eggs and small ducklings need protein in their diet. Boreal lakes are typically barren and invertebrate-poor. A newly published review article emphasizes the need to carefully deliberate when considering the introduction of fish in wetlands where they do not originally belong or that are established specifically for ducks.
Fish modify the structure of aquatic invertebrate communities, and thus the structure, abundance and diversity of invertebrate communities differ between lakes with and without fish. Fish predate especially large invertebrates that typically are the top predators of invertebrate communities. Therefore the community structure is skewed towards smaller species in lakes with fish.
Invertebrates living among vegetation are better protected from fish predation and thus predation is higher in open water. This means that fish compete especially with those duck species that forage in the open water (e.g. teal), while species foraging among vegetation are less affected (e.g. mallard).
The review article clearly showed that the food competition caused by fish is harmful for breeding ducks. However, the situation is not always clear, because fish and duck abundances are typically limited by the same environmental key factors such as lake productivity. Thus both ducks and fish can be abundant in lakes rich in invertebrate and vegetative food material. But this competitive set-up is emphasized in barren lakes.
Researchers in Finland introduced fish to lakes that had become fishless due to acidification. Monitoring showed that the lake use by common goldeneye broods declined after these introductions. Pairs on the other hand continued using the lakes as before. The difference is suggested to be caused by the foraging manners of ducks at different life stages. Adult ducks find their food from the lake benthos, while ducklings and fish concentrate on competing over nektonic invertebrates. Competition theory is also supported by other studies: ducklings need to use more time foraging in lakes with fish, and still seem to grow slower than in fishless lakes.
The fish experiment was performed in the opposite way in Sweden, where fish were eradicated from certain lakes. Researchers found that all invertebrate groups became more abundant and goldeneye brood numbers increased.
In Finland lakes have been recovering from acidification and this has positively reflected to fish populations. The recovery of fish might affect the breeding success of ducks in boreal lakes, especially concerning breeding goldeneyes.
The competition between fish and ducks is asymmetric in the sense that fish will affect ducks, but ducks do not affect fish. Fish are present in the lakes year-round and if times are thin, the fish just grow slower. They also affect the invertebrate populations of lakes. Ducklings will also grow slower in bad times, but their mortality increases rapidly if food becomes scarce. The effect of ducks on invertebrates is also milder. Thus fish should not be introduced to wetlands established especially for ducks.
Nummi, P., Väänänen, V.-M., Holopainen S. & Pöysä H. 2016. Duck–fish competition in boreal lakes – a review. – Ornis Fennica 93: 67-76.
Gunnarsson, G., Elmberg, J., Sjöberg, K., Pöysä, H. & Nummi, P. 2006. Experimental evidence for density-dependent survival in mallard (Anas platychynchos) ducklings. — Oecologia 149: 203-213
Nummi, P., Väänänen, V-M., Rask, M., Nyberg, K. & Taskinen, K. 2012. Competitive effects of fish in structurally simple habitats: perch, invertebrates, and goldeneye in small boreal lakes. — Aquatic Sciences 74: 343-350.